The Versatility of Ethylene Glycol to Tune the Dimensionality and Magnetic Properties in DyIII-Anilato-Based Single-Ion Magnets

We exploit the high versatility of the solvent ethylene glycol (eg = CH2OH-CH2OH) acting as a ligand with three different coordination modes: terminal (κO), chelate (κ2O,O′), and bridge (1κO,2κO′) to prepare a novel family of six different coordination polymers with DyIII and three different anilato ligands (3,6-disubstituted-2,5-dihydroxy-1,4-benzoquinone dianion = C6O4X22–, with X = H, Cl, and Br). With the X = H derivative (dhbq2–), we have prepared [Dy2(dhbq)3(eg)2(μ-eg)]·4eg·2H2O (1), a 3D diamond-like network with a chelate and bridging eg molecules. With the X = Cl derivative (chloranilato), we have prepared [Dy2(C6O4Cl2)3(eg)4]·2eg·H2O (2) and [Dy2(C6O4Cl2)3(μ-eg)(H2O)4]·2eg·7H2O (3). Compound 2 has a 2D (6,3)-gon brick-wall lattice and contains a chelate and a terminal eg molecule. Compound 3 has a 3D diamond-like topology as 1, although now the chelate eg has been replaced by two water molecules. Finally, with the X = Br derivative (bromanilato), we have obtained [Dy2(C6O4Br2)3(eg)2(CH3OH)2]·2eg·4CH3OH (4), [Dy2(C6O4Br2)3(eg)4]·4eg (5), and [Dy2(C6O4Br2)3(eg)3(H2O)]·2eg·H2O (6). Compound 4 has a 2D (6,3)-gon herringbone topology and contains a chelate eg and a MeOH molecule. Compounds 5 and 6 have a 2D (6,3)-gon brick-wall topology with a chelate and a terminal eg molecules (in 5 and in one of the two independent Dy centers of 6). The other Dy center in 6 has a chelate eg and a water molecule. All the compounds show slow relaxation of the magnetization at low temperatures (in compounds 1, 2, and 5 with no applied DC field). The magnetization of compounds 1–6 relaxes through Orbach and direct mechanisms when a DC field is applied and through an Orbach and/or quantum tunneling mechanism when no DC field is applied.


■ INTRODUCTION
Two of the hottest topics in material science nowadays are (i) porous crystalline coordination polymers (CP), best known as metal organic frameworks (MOFs), 1,2 and (ii) single-molecule (or single-ion) magnets (SMM and SIM). 3−7 Combining these two topics is, therefore, a very appealing challenge, since MOFs behaving as SIM may find specific applications in different fields as magnetic-based sensors of different chemical species as gases, solvents, and contaminants. 8−10 Although there are many examples of magnetic MOFs, 11−13 very few show a slow relaxation of the magnetization, behaving as SMM or SIM. 14,15 In most of these cases, the presence of a DC magnetic field is required to suppress the fast relaxation of the magnetization through a quantum tunneling mechanism (field-induced SMM or SIM).
A very promising and recent strategy to obtain MOFs with SIM or FI-SIM behavior consists of combining lanthanoid metals ions, mainly Dy III , with different bridging ligands to build extended structures. When the bridging ligands are poor magnetic couplers, the Ln(III) centers are well isolated (since the 4f orbitals are deep) and become good candidates to behave as SIM or FI-SIM. We have selected the Dy III ion since it presents an oblate single-ion electron density and shows a strong axial crystal field below and above the equatorial plane, stabilizing the largest m J and maximizing the uniaxial anisotropy. 16 As bridging ligands, we have selected anilato-type ligands (3,6-disubstituted-2,5-dihydroxy-1,4-benzoquinone dianion = C 6 O 4 X 2 2− , Scheme 1), as they are a large family of ligands (X = H, F, Cl, Br, I, NO 2 , Cl/CN, CH 3 , t-but, etc.) 17,18 that can be used to construct two-and three-dimensional (2D and 3D) MOFs with different transition metal ions 19−21 or lanthanoids. 22 −37 These studies show that anilato-type ligands usually coordinate in a bis-bidentate way (Scheme 1). Transition metal ions are usually hexacoordinated by three chelating anilato ligands, giving rise to 2D lattices with a honeycomb (6,3)-gon topology. 19−21 In contrast, lanthanoids are usually octa-or nona-coordinated and appear surrounded by three (four in a few cases) chelating anilato ligands, the remaining coordination positions being occupied by solvent molecules as H 2 O, dimethylformamide (dmf), dimethyl-sulfoxide (dmso), etc. with a high coordination ability toward Ln(III) ions. 38,39 The key role of these coordinated solvent molecules on structural aspects as the coordination number, geometry, and final structure, has been very recently pointed out by some of us. 26,27,29−33 Despite the larger coordination number, when the Ln(III) ions are surrounded by three chelating anilato ligands, the most common topology is also the (6,3)-gon. 24 Nevertheless, the presence of the solvent molecules produces a distortion of the 6-membered rings that appear as distorted hexagons 24,26,29,40,41 or even as rectangles, giving rise to brickwall 22, 23,30,32,37 and herringbone 26,28,29,31,37 lattices. In the few cases where the Ln(III) ions are surrounded by four anilato ligands, the two most common topologies are the 2D (4,4)gon square 27,36,42,43 and the 3D (6,4)-gon diamond-like. 24,37 The use of anilato ligands to construct these 2D and 3D lattices is based, among other properties, on their capacity to act as bis-bidentate bridges, connecting two metal atoms (Scheme 1d) and their capacity to magnetically isolate lanthanoid ions (as a result of the negligible overlap with the 4f orbitals). This last property has very recently started to be exploited to prepare MOFs with SIM and SMM behavior. 28,31−35, 42 Although the number of different solvent molecules used to complete the coordination sphere of the Ln(III) ions is quite high, 33 all of them are rather simple monodentate molecules as H 2 O, ethanol, dimethylformamide (dmf = Me 2 NCHO), dimethyl sulfoxide (dmso = Me 2 SO), formamide (fma = NH 2 CHO), dimethylacetamide (dma = Me 2 NC(Me) O), and hexamethylphosphoramide (hmpa = (Me 2 N) 3 PO). To date, no attempt has been done to prepare anilato-based lattices with potentially bridging solvent molecules as ethylene glycol (eg = CH 2 OH-CH 2 OH). Here we show how the coordination versatility of eg may generate up to six different extended lattices when used with Dy III and three different anilato ligands of the type (C 6 O 4 X 2 ) 2− with X = H (dhbq 2− ), Cl (C 6 O 4 Cl 2 2− ), a n d B r ( C 6 O (6), also with a 2D brick-wall structure. Herein, we show how slight changes in the synthetic conditions may lead to important changes in the structures and in the magnetic properties, thanks to the coordination versatility of the solvent ethylene glycol, that may act as bridging, chelate, and/or monodentate ligand.
Experimental Section. Starting materials: All the chemicals and solvents were of reagent grade and used as received from commercial sources without further purification.
Given the limited number of crystals obtained in all cases, we checked the unit cell parameters of at least ten single crystals for each compound and verified that all the crystals used for the magnetic characterization for each compound had the same color and shape.
Synthesis of [Dy 2 (dhbq) 3 (eg) 2 (μ-eg)]·4eg·2H 2 O (1). Red plate-shape single crystals of compound 1 were obtained by carefully layering, at room temperature, a top solution of 2,5-dihydroxi-1,4-benzoquinone, H 4 C 6 O 4 (2.8 mg, 20 μmol) in 5 mL of methanol, a middle phase (5 mL) of ethylene glycol, and a bottom solution of Dy(NO 3 ) 3 ·5H 2 O (8.8 mg, 20 μmol) in 5 mL of ethylene glycol. The tube was sealed and allowed to stand for about 3 weeks. Suitable crystals for X-ray diffraction were freshly picked and covered with paratone oil in order to avoid solvent loss to be characterized by single crystal X-ray diffraction.
Synthesis of [Dy 2 (C 6 O 4 Cl 2 ) 3 (eg) 4 ]·2eg·H 2 O (2). Violet block-shaped single crystals of compound 2 were obtained by carefully layering, at room temperature, a top solution of chloranilic acid, H 2 C 6 O 4 Cl 2 (4.2 mg, 20 μmol) in 5 mL of methanol, a middle phase (5 mL) of ethylene glycol, and a bottom solution of Dy(NO 3 ) 3 ·5H 2 O (8.8 mg, 20 μmol) in 5 mL of ethylene glycol. The tube was sealed and allowed to stand for about 2 weeks. Suitable crystals for X-ray diffraction were freshly picked and covered with paratone oil in order to avoid solvent loss to be characterized by single crystal X-ray diffraction.
Synthesis of [Dy 2 (C 6  sealed and allowed to stand for 11 days. Suitable crystals for Xray diffraction were freshly picked and covered with paratone oil in order to avoid solvent loss to be characterized by single crystal X-ray diffraction.
When the previous solution is left undisturbed for more than 3 weeks, the pink plates become violet block-shaped single crystals of compound 6. Suitable crystals for X-ray diffraction were freshly picked and covered with paratone oil to avoid solvent loss to be characterized by single crystal X-ray diffraction. Eight months later, in the previous tube, the crystals still have the same structure as compound 6.
Physical Measurements. Magnetic susceptibility of polycrystalline samples of complexes 1−6 was measured on a Quantum Design MPMS-XL-5 SQUID susceptometer with an applied magnetic field of 0.1 T in the temperature range 2−300 K. AC susceptibility measurements were performed on the same samples with an oscillating magnetic field of 0.8 mT in the frequency range 10−10000 Hz at low temperatures with different applied DC fields with a Quantum Design PPMS-9 equipment. The susceptibility data were corrected for the sample holder, and the corresponding diamagnetic contribution was evaluated using Pascal's constants. 44 Crystallographic Data Collection and Refinement. Single crystals of compounds 1−6 were mounted on a mylar loop using a viscous hydrocarbon oil to coat the crystal and then transferred directly to the cold nitrogen stream for data collection. X-ray data were collected at 120 K on a Supernova diffractometer equipped with a graphite monochromated Enhance (Mo) X-ray Source (λ = 0.71073 Å). The program CrysAlisPro, Oxford Diffraction Ltd., was used for unit cell determinations and data reduction. 45 Empirical absorption correction was performed using spherical harmonics, implemented in the SCALE3 ABSPACK scaling algorithm. Compound 1, 3, 5, and 6 crystallize in the triclinic P1̅ space group, whereas compounds 2 and 4 crystallize in the monoclinic P2 1 /n and P2 1 /c space groups, respectively (Tables 1 and 2). Crystal structures were solved with the XT 46 structure solution program using the Intrinsic Phasing solution method and by using Olex2 47 as the graphical interface. The model was refined with version 2017/1 of XL 48 using Least Squares minimization. All non-hydrogen atoms were refined anisotropically. Hydrogen atom positions were calculated geometrically and refined using the riding model. Most hydrogen atom positions were calculated geometrically and refined using the riding model, but some hydrogen atoms were refined freely.
A summary of the data collection and structure refinements for compounds 1−6 is given in Tables 1 and 2. CCDC 2222691−2222696 contain the supplementary crystallographic data for compounds 1−6, respectively. Tables S1−S12 in the Supporting Information contain the bond distances and angles for compounds 1−6.

■ RESULTS AND DISCUSSION
Syntheses of the Compounds. The synthesis of compounds 1−6 was performed using a slow diffusion technique that allows the preparation of good quality single crystals. As can be seen in the Experimental Section and in Table 3, the synthetic conditions are the same for compounds 1, 2, and 4. The only change is the anilato derivative (X = H in 1, X = Cl in 2, and X = Br in 4). On the other side, if we change the MeOH solvent by a 1:1 mixture of H 2 O and MeOH in the top layer, we obtain compound 3 (for X = Cl) and compounds 5 and 6 (for X = Br). For X = H, the change of the solvent does not produce any change in the final product. It is to be noted that compounds 5 and 6 are prepared with exactly the same conditions and ligands (X = Br). The only difference is the crystallization time: compound 5 is obtained for short crystallization periods (1 week), whereas compound 6 is obtained by leaving the synthesis of 5 for longer crystallization times (between 3 weeks and at least eight months). We can, therefore, conclude that compound 6 is the thermodynamic phase, whereas 5 is the kinetic one. As we will discuss below, the differences between 5 and 6 are, on the one hand, the change of one monodentate eg molecule in 5 by a water molecule in half of the Dy III ions in 6. On the other hand, in 5 there is one independent Dy III atom, while in 6 there are two crystallographically independent Dy III centers.
The Dy−O bond distances are also similar to those found in other related structures with nona-coordinated Dy III and anilato ligands. 27,30,32,34,36 O (3). Compound 3 crystallizes in the triclinic P1̅ space group (Table 1). Its structure is very similar to that of compound 1. Besides the obvious change in the ligand (dhbq 2− in 1vs chloranilato in 3), the only differences are the presence of two coordinated water molecules in 3, instead of a chelate eg molecule in 1 and the crystallization solvent molecules (four eg and two H 2 O molecules in 1vs two eg and seven H 2 O molecules in 3). The asymmetric unit of 3 contains one Dy III ion, half coordinated eg molecule, three halves chloranilato ligands, and two coordinated water molecules. There are also three and a half water and one eg crystallization molecules ( Figure S2). The coordination number and geometry (Table  4), the connectivity, and the topology (diamond like) are the same in 1 and 3 ( Figure S2). As expected, the average Dy− O anilato bond distances are slightly longer in 3 (2.413 Å,   The minimum values are indicated in bold. EP-9 = Enneagon; OPY-9 = Octagonal pyramid; HBPY-9 = Heptagonal bipyramid; JTC-9 = Triangular cupola (J3) = trivacant cuboctahedron; JCCU-9 = Capped cube (Elongated square pyramid, J8); CCU-9 = Capped cube; JCSAPR-9 = Capped square antiprism (Gyroelongated square pyramid J10); CSAPR-9 = Capped square antiprism; JTCTPR-9 = Tricapped trigonal prism (J51); TCTPR-9 = Tricapped trigonal prism; JTDIC-9 = Tridiminished icosahedron (J63); HH-9 = Hula-hoop; MFF-9 = Muffin. crystallization solvent molecules, and (iii) the relative position of the coordinated anilato ligands in the coordination environment ( Figure 3b). Thus, in 4 the two oxygen atoms of the three chelate anilato ligands occupy one coordination position in the equatorial plane and the other in one of the two triangular faces (eq-tr). This spatial orientation of the bromanilato ligands also results in planar layers ( Figure S3) with rectangular cavities (as in 2), but now the rectangles show a different relative orientation, giving rise to a herringbone structure in the layers (Figure 3d). Note that in compound 2, only two of the three anilato ligands occupy one position in the equatorial plane and the other in one triangular face (eq-tr). The third anilato (O12/O16) occupies two positions in one triangular face (tr-tr) ( Figure 2b). As in 2, the layers are packed eclipsed along the a direction, giving rise to rectangular channels (that contain the crystallization methanol molecules, Figure S3).  4 ]·4eg (5). Compound 5 crystallizes in the triclinic P1̅ space group (Table 2). Its asymmetric unit contains one Dy III ion, three-halves bromanilato ligands, and one chelate and one terminal coordinated eg molecules ( Figure S4). There are also two eg crystallization molecules. The total formula is, therefore, [Dy 2 (C 6 O 4 Br 2 ) 3 (eg) 4 ]·4eg. Compound 5 is very similar to compound 2. In fact, besides the change of chloranilato in 2 by bromanilato in 5, the only difference is the coordination geometry (distorted tricapped trigonal prims in 2vs distorted capped square antiprism in 5, Table 4 and Figure S4). Despite this difference, the connectivity, the shape of the cavities (rectangles, Figure S5), and their disposition (brick-wall, Figure S4) are the same in both compounds. The layers are also packed in an eclipsed way forming channels parallel to the a direction ( Figure S5). The average Dy−O anilato and Dy−O eg bond distances in 5 (2.399 and 2.415 Å, respectively, Table  S9) are also very similar to those observed in 2 (2.408 and 2.428 Å, respectively). 2 O (6). Compound 6 crystallizes in the triclinic P1̅ space group (Table 2). Its asymmetric unit contains two Dy III ions, two complete and two half bromanilato ligands, two chelate and one terminal coordinated eg molecules, and one coordinated water molecule (Figure 4). There are also two eg and one water crystallization The structure of compound 6 is similar to 2 and 5, although there are some slight differences. In 6 three are two independent Dy III centers. Dy1 is surrounded by three bromanilato ligands one chelate and one terminal eg molecule. Dy2 has a coordinated water molecule instead of the terminal eg molecule (Figure 4a). Both Dy III centers show a capped square antiprism geometry ( Figure  4b), as observed in 5. Each Dy1 center is connected, through three bromanilato bridges, to two Dy2 and one Dy1 centers, whereas each Dy2 is connected to two Dy1 and one Dy2 (Figure 4c). Compound 6 also forms parallel layers with rectangular rings ( Figure S6) with a brick-wall structure ( Figure 4d). As in 5, the layers are packed in an eclipsed way, leading to rectangular channels along the a direction ( Figure  S6) As mentioned above, compound 5 transforms into compound 6 after a few weeks inside the tube. As a consequence, a new structure was obtained with two crystallographically independent Dy III atoms. In half of them,  Role of the Solvent Coordination Mode on the Structure of Compounds 1−6. The analysis of the structures of compounds 1−6, shows different conclusions: (i) The two 3D structures (compounds 1 and 3) present a bridging eg molecule that constitutes the fourth connection to obtain the diamond-like structure. In compound 1, there is a chelate eg molecule occupying the two remaining coordination positions, whereas in 3, these positions are occupied by two water molecules, probably due to a reduction in the free space when passing from X = H (in 1) to X = Cl (in 3). (ii) Compounds 2, 5, and 6 show the same brick-wall 2D topology and also the same coordination environment in the Dy centers (one chelate and one terminal eg molecule). In these three compounds, the chelate eg molecule reduces the available space and only allows the coordination of a terminal eg molecule (or a H 2 O molecule in one of the two Dy atoms in 6). (iii) Compound 4, although it also has a chelate eg molecule, is the only one with a herringbone structure topology. This original topology is due to the different disposition of the chelate eg molecule. Thus, in 4, the chelate eg molecule occupies two positions in the same triangular face with the methanol molecule in the other triangular face, leading to the formation of a herringbone structure. This disposition contrasts with that in the brick-wall structures (2, 5, and 6), where the chelate eg occupies a position in the equatorial plane and a position in one triangular face, with the extra molecule in the same triangular phase. In summary, the bridging coordination mode of eg leads to 3D structures with a diamond-like topology, whereas the chelating coordination mode of eg plus a monodentate ligand leads to 2D structures with the 6,3-gon topology.

Structure of [Dy 2 (C 6 O 4 Br 2 ) 3 (eg) 3 (H 2 O)]·2eg·H
Magnetic Properties. The magnetic properties of compounds 1−6 are, in general, quite similar, although there are important differences due to the presence of different coordination environments. The product of the molar magnetic susceptibility per formula unit (two Dy III ions) times the temperature (χ m T) shows in all cases a room temperature value of 28.2−28.7 cm 3 K mol −1 ( Figure S7). This value is close to the expected one for two independent Dy III ions (28.34 cm 3 K mol −1 ), 50 in agreement with the good magnetic isolation provided by the anilato ligands when connecting Ln(III) ions. 33 When the temperature is decreased, χ m T remains almost constant down to ca. 100 K and below this temperature, it shows a progressive decrease attributed to the depopulation of the excited levels that appear due to the ligand field ( Figure S7). 50 This good magnetic isolation and the behavior observed in other Dy III -anilato lattices 28,31,32,34 prompted us to perform AC susceptibility measurements to check a possible single-ion magnet (SIM) or field-induced SIM behavior in compounds 1−6.
When no DC field is applied, the AC susceptibility measurements show, in compounds 1, 2, and 5, the presence of a frequency dependent out of phase signal (χ" m ) at low temperatures with a maximum at high frequencies (HF relaxation process, Figure S8). Note that, although in compound 1 the maximum of χ" m is located above 10 kHz, Figure 5. (a−f) Frequency dependence of χ" m at different temperatures for compounds 1−6 with different applied DC fields. Solid lines are the best fit to a Debye model with one (in 3 and 4) or two (in 1, 2, 5, and 6) relaxation processes. the highest frequency we can achieve, we can clearly see a χ" m signal at high frequencies. When a DC field is applied, we can observe in compounds 1, 2, 5, and 6 the appearance of two maxima: one at low frequencies (LF relaxation process) and one at high frequencies (HF relaxation process, Figure S8). In compounds 3 and 4, the application of a DC field gives rise to the appearance of a frequency dependent signal with a maximum at around 3 × 10 2 Hz ( Figure S8). In all cases (except for the HF signal in compound 5), the maxima of the χ" m signals increase in intensity and shift to lower frequencies with increasing the DC field up to a certain DC field. Above this value, the maxima shift to higher frequencies and decrease in intensity ( Figure S8). In compound 5, the HF signal only shows the decrease in intensity and the shift to higher frequencies as the DC field increases.
By fitting the frequency dependence of the χ" m signal to the Debye model with one (in 3 and 4) 51 or two (in 1, 2, 5, and 6) 52 relaxation processes, we can obtain the relaxation times for the LF (τ LF ) and HF (τ HF ) processes at each applied DC field (solid lines in Figure S8). Note that this double relaxation process has been observed in other Dy III -containing compounds, 53 and, since there is only one independent Dy III ion in 1, 2, and 5, it has to be attributed to the presence of two relaxation pathways via excited states. 54 Only in compound 6 the presence of two relaxation times may be attributed to the presence of two crystallographically independent Dy III centers. Note that the relaxation times obtained at high temperatures in some cases have to be taken with caution since the maxima in the χ" m vs frequency plot cannot be observed.
As expected, the relaxation times increase as the DC field increases, reach a maximum at intermediate fields, and show a decrease for higher DC fields ( Figure S9). In order to perform a detailed study of the χ" m signal as a function of the temperature, we have fixed the DC fields to the values where the relaxation time shows a maximum. Additionally, we have also performed the study of the χ" m signal as a function of the temperature with zero DC field in those compounds that show a χ" m signal with zero DC field (1, 2, and 5).
The frequency dependence of χ" m at different temperatures for compound 1 with zero DC field shows a signal whose maximum appears above 10 kHz (the maximum frequency that we can reach, Figure S10). As the temperature increases, the signal moves to higher frequencies, and, thus, we cannot perform a detailed study of this signal for H DC = 0 mT. When a DC field of 60 mT is applied, there are two clear maxima that shift to higher frequencies as the temperature increases ( Figure  5a). The χ" m signal can be fitted in the temperature range 2.0− 3.8 K with a Debye model considering two relaxation processes: one at low frequencies, LF, and a second one at high frequencies, HF (solid lines in Figure 5a). Finally, we have also performed the AC measurements with an applied DC of 100 mT ( Figure S10). These measurements show very clearly how the LF signal shifts to high frequencies as the temperature increases. The corresponding fit to the Debye model for two relaxation processes is shown as solid lines in Figure S10.
Compound 2 shows a similar behavior (Figure 5b): There is only one relaxation process when H DC = 0 mT ( Figure  S11.left), although now the maximum appears below 10 kHz and, therefore, we can fit it to a single relaxation process with Figure 6. Arrhenius plots of the relaxation times of compounds 1−6 with different applied DC fields. Solid lines are the best fits to the general model (eq 1) with the parameters shown in Table 5. the Debye model (solid lines in Figure S11.left). When a DC field of 50 or 100 mT is applied ( Figure S11.right and Figure  5b, respectively) we can observe two maxima that can be well reproduced (solid lines in Figure 5b) with a Debye model considering two relaxation processes.
Compounds 3 and 4 show no χ" m signal for H DC = 0 mT, but for H DC = 60 mT (for 3) and 70 mT (for 4) they show a χ" m signal at low temperatures that can be well reproduced with the Debye model for a single relaxation process (solid lines in Figure 5c for 3 and Figure 5d for 4).
Compound 5 behaves as 2: it shows one relaxation process for H DC = 0 mT ( Figure S12) with maxima below 10 kHz that can be fitted to a Debye model with a single relaxation process (solid lines in Figure S12). When a DC field of 50 mT is applied (Figure 5e), we observe two maxima that can be well reproduced (solid lines in Figure 5e) with the Debye model considering two relaxation processes.
Finally, compound 6 shows no χ" m signal for H DC = 0 mT, but when a DC field of 60 mT is applied (Figure 5f), we observe two broad maxima that can be well reproduced (solid lines in Figure 5f) with the Debye model considering two relaxation processes.
The fit of the χ" m signals shown in Figure 5 (and Figures S10−S12) provides the relaxation times of each process as a function of the temperature. The Arrhenius plots (Ln τ vs 1/T) of the relaxation times for the different processes with different applied DC fields are shown in Figure 6.
In general, the relaxation times show a straight line behavior at high temperatures (Orbach activated mechanism) with a curvature at lower temperatures (direct mechanism, that appears when H DC ≠ 0). Thus, these Arrhenius plots can be fitted to a general model including quantum tunneling (QTM, first term), Orbach (second term), Raman (third term), and direct (fourth term): 55 In all cases, when H DC ≠ 0, the relaxation times can be well fitted with direct and Orbach contributions with the parameters shown in Table 5. In compounds 2 for H DC = 0, the relaxation time of the only signal observed can be fitted to a simple quantum tunneling mechanism, whereas in compound 5 the HF signal for H DC = 0 and 50 mT can be fitted with a quantum tunneling plus an Orbach mechanism. Note that the temperature ranges are limited by the possibility to obtain reliable fits of the frequency dependence of the χ" m signal.

■ CONCLUSIONS
We have shown the versatility of ethylene glycol as a solvent and as a ligand to prepare coordination polymers with anilatotype ligands and Dy III . The possibility of ethylene glycol to act as monodentate (eg-κO), bidentate chelate (eg-κ 2 O,O'), and bidentate bridge (μ-eg-1κO,2κO') has been exploited in this work to prepare up to six different Dy-anilato coordination polymers where the Dy III are surrounded by three chelate anilato ligand that connect each Dy III ion to three neighboring Dy III ions. Additionally, the Dy III ions complete their nonacoordination with chelate, terminal, and/or bridging eg molecules, H 2 O, and/or MeOH.
Thus, with the 2,5-dihydroxy-1,4-benzoquinone dianion (dhbq 2− ), we have prepared a 3D polymer with a diamondlike topology where the Dy III ions complete their coordination with a chelate eg molecule and a bridging eg that forms the fourth Dy−Dy connection in the diamond-like topology (compound 1). When using the 3,6-dichloro-2,5-dihydroxy-1,4-benzoquinone dianion (chloranilato), we obtain two different coordination polymers depending on the synthetic conditions: a 2D (6,3)-gon brick-wall topology where the Dy III ions complete their coordination with a chelate and a terminal eg molecule (compound 2) and a 3D diamond-like topology similar to that of compound 1, although now the chelate eg has been replaced by two water molecules (compound 3). Finally, when using the 3,6-dibromo-2,5-dihydroxy-1,4-benzoquinone dianion (bromanilato), we obtain up to three different coordination polymers: a 2D (6,3)-gon herringbone topology where the Dy III ions complete their coordination with a chelate eg molecule and a MeOH molecule (compound 4), a 2D (6,3)-gon brick-wall topology where the Dy III ions complete their coordination with a chelate and a terminal eg molecule (compound 5), and a 2D (6,3)-gon brick-wall topology with

Crystal Growth & Design
two independent Dy III ions: one with a chelate and a terminal eg molecule and the second one with a chelate eg molecule and a water molecule (compound 6). The coordination environments of the Dy III ions are tricapped trigonal prisms in compounds 1−4 and capped square antiprisms in 5 and 6. The competition of the coordination ability of the three coligands used (ethylene glycol, H 2 O and MeOH) results in important changes in the dimensionality and final structures in compounds 1−6.
Thus, if we add water in the synthesis of compound 2, we observe the formation of compound 3 where each Dy III ion has two coordinated water molecules (instead of a chelate eg, as observed in 2). This change reduces the steric hindrance around the Dy III centers and allows the formation of an ethylene glycol bridge connecting two Dy III ions that results in a 3D structure in 3 (in contrast to the 2D structure in 2). In compounds 4−6, prepared with bromanilato, we also observe an important influence of the water molecules. When no water molecule is added, we obtain compound 4, that contains a coordinated MeOH molecule (besides a chelate eg ligand). When we add water, the MeOH molecule is probably replaced initially by a water molecule leading to a change in the coordination geometry (from tricapped trigonal prismatic to capped square antiprismatic). This change in the geometry reduces the steric hindrance around the Dy III center and allows the inclusion of a terminal eg ligand, as observed in the kinetic phase (compound 5) and of a water molecule in half of the Dy III centers in the thermodynamic phase (compound 6).
The magnetic properties of the six coordination polymers are the expected ones for isolated Dy III ions. Interestingly, the six compounds present slow relaxation of the magnetization (in compounds 1, 2, and 5 even with no applied DC field). When a DC field is applied, compounds 1, 2, 5, and 6 show two different relaxation processes (high and low frequency processes), whereas compounds 3 and 4 only show one relaxation process. We have studied the frequency dependence of the χ" m signal at different DC fields and determined the optimal DC fields to perform a detailed study of the χ" m signal as a function of the temperature. This study shows that the magnetization of all compounds relaxes through direct and Orbach mechanisms with energy barriers in the range 16−49 K when a DC field is applied and through an Orbach and/or quantum tunneling mechanism when no DC field is applied.
Although there is no a linear correlation, if we compare the U eff values for intermediate DC fields (50−70 mT, Table 5) with the distortion from the regular geometry, measured by the shape parameter (Table 4), we can observe an increase in U eff as the distortions from the ideal geometry increase. Thus, the more regular compounds (1 and 2) show the lower U eff values (25 and 19 K, respectively), whereas the most distorted compounds (3−6) show the highest U eff (37, 29.4, 48, and 47 K, respectively). This trend indicates that the higher the distortions of the coordination geometry, the higher is the energy barrier and further confirms that the use of chelating and bulky solvent molecules may be a very adequate strategy to increase the energy barrier in these Dy III -based 2D and 3D SIMs.
In summary, we have shown the versatility of the solvent and ligand ethylene glycol to prepare coordination polymers with different dimensionalities (2D and 3D) using Dy III and three different anilato-type ligands (dhbq 2− , chloranilato, and bromanilato) by slightly changing the synthetic conditions. Of course, this result opens the way to prepare many other coordination polymers with other interesting magnetic and optical properties by simply changing the Dy III ion by other lanthanoid ions and transition metal ions. We are also extending this study with Dy III (and other ions) using other symmetric and asymmetric anilato-type ligands with X = F, I, CH 3